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digitalmars.D.bugs - [Issue 12133] New: Relaxed read-modify-write for shared lvalues

reply d-bugmail puremagic.com writes:
https://d.puremagic.com/issues/show_bug.cgi?id=12133

           Summary: Relaxed read-modify-write for shared lvalues
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: druntime
        AssignedTo: nobody puremagic.com
        ReportedBy: stanislav.blinov gmail.com



15:31:31 PST ---
TDPL prohibits using operators on shared variables. I.e. this:

shared int i;
++i;

is illegal. It is stated that core.atomic should be used instead:

shared int i;
atomicOp!"+="(i, 1);

However, there are cases when this can introduce unnecessary performance loss:
e.g. private shared variables that are only ever accessed under a lock.

I'm proposing to add two functions to core.atomic to allow for operations on
shared variables without atomicOp()'s performance penalties:

1) localOp:

performs and acquire-op-release, e.g.:

shared int i;
auto v = localOp!"+="(i, 1);

Useful when there's still need for ordering. Though I don't really like the
name "localOp".

2) assumeLocal:

converts shared lvalue to a non-shared lvalue, e.g:

shared int i;
++assumeLocal(i);

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Feb 11 2014
next sibling parent d-bugmail puremagic.com writes:
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Andrej Mitrovic <andrej.mitrovich gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich gmail.com



15:36:23 PST ---


 2) assumeLocal:
 
 converts shared lvalue to a non-shared lvalue, e.g:
 
 shared int i;
 ++assumeLocal(i);


shared int i;
++cast()i;

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15:39:09 PST ---



 shared int i;
 ++cast()i;


cast()i would cast away everything, including const, silently :)

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15:49:51 PST ---



 

 shared int i;
 ++cast()i;


 
 cast()i would cast away everything, including const, silently :)


Oops. Maybe something like:

-----
template Unshared(T)
{
         static if (is(T U == shared inout const U)) alias Unshared = U;
    else static if (is(T U == shared inout       U)) alias Unshared = U;
    else static if (is(T U == shared       const U)) alias Unshared = U;
    else static if (is(T U == shared             U)) alias Unshared = U;
    else                                             alias Unshared = T;
}

ref unshared(T)(ref T var)
{
    return *cast(Unshared!T*)&var;
}

void main()
{
    shared int x;
    ++unshared(x);
    assert(x == 1);
}
-----

Unshared code based off of Unqual.

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15:50:29 PST ---


 template Unshared(T)
 {
          static if (is(T U == shared inout const U)) alias Unshared = U;
     else static if (is(T U == shared inout       U)) alias Unshared = U;
     else static if (is(T U == shared       const U)) alias Unshared = U;
     else static if (is(T U == shared             U)) alias Unshared = U;
     else                                             alias Unshared = T;
 }


Uhh that should teach me to copy-paste, the RHS should simply strip shared, not
strip all the qualifiers.

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15:51:03 PST ---


 Uhh that should teach me to copy-paste, the RHS should simply strip shared, not
 strip all the qualifiers.


Fixed: 

template Unshared(T)
{
         static if (is(T U == shared inout const U)) alias Unshared = inout
const U;
    else static if (is(T U == shared inout       U)) alias Unshared = inout U;
    else static if (is(T U == shared       const U)) alias Unshared = const U;
    else static if (is(T U == shared             U)) alias Unshared = U;
    else                                             alias Unshared = T;
}

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15:57:16 PST ---
Something a bit less involving :)

ref auto assumeLocal(T)(ref shared T var)  trusted pure nothrow
{
    return *cast(T*)&var;
}

I should've probably noted that I already have implementations for both
functions in mind, just not in the state of pull request yet.

But comments, suggestions, objections and improvements are key!

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16:01:43 PST ---
See also the discussion thread:

http://forum.dlang.org/thread/moyyibrpnnmrrovylkui forum.dlang.org

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Stanislav Blinov <stanislav.blinov gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull



22:15:42 PST ---
Pull request for assumeLocal():
https://github.com/D-Programming-Language/druntime/pull/724

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Feb 14 2014
prev sibling parent d-bugmail puremagic.com writes:
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Andrei Alexandrescu <andrei erdani.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei erdani.com



PDT ---
localOp should be atomicOpRelaxed

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Mar 15 2014